For the sake of safety, mobile telephones installed in vehicles are now commonly adapted to operate in a so-called hands-free mode, i.e. without the need to move the handset from the cradle. The driver is thus able to make and receive calls safely without having to physically pick up and hold the handset. Usually an incoming call can be answered by pressing one of the keys on the handset, the keystroke producing an off-hook condition enabling the call to proceed.
As used herein, the term "off-hook condition" means the telephone is in a state of answering or making a telephone call.
In the further interests of road safety even the keystroke can be avoided if the mobile telephone is adapted to produce an off-hook condition automatically in response to an incoming call. However this arrangement has the disadvantage that if the subscriber is absent from the telephone at the time of an incoming call, the telephone will nevertheless go off-hook automatically. Apart from the frustration caused to the caller by the unnecessary expense of a connected call, the transceiver of the mobile telephone will become fully operational with the attendant drain on the vehicle power supply.